As a steering apparatus of an automobile, conventionally, a steering apparatus having a rack-and-pinion steering gear unit has been widely used. FIGS. 11 to 13 illustrate an example of a conventional structure of such a steering apparatus. This steering apparatus, the overall configuration of which is shown in FIG. 11, has a configuration capable of giving a desired steering angle to the left and right wheels to be steered (the front wheels) by converting the rotational motion of a steering wheel 1 operated by the driver into a linear motion using a rack-and-pinion steering gear unit 5. More specifically, the steering wheel 1 is fixed to the rear end portion of a steering shaft 2 in order to realize this configuration. In addition, the front end portion of this steering shaft 2 is connected to the base end portion of the pinion shaft 6 of the steering gear unit 5 via a pair of universal joints 3, 3 and an intermediate shaft 4. Furthermore, the base end portions of a pair of tie rods 8, 8 respectively connected to the left and right wheels to be steered are connected to the axial end portions of the rack shaft 7 that is engaged with the pinion shaft 6.
The steering gear unit 5 includes a gear housing 9, the pinion shaft 6, the rack shaft 7, and a pressing means 10 as shown in detail in FIGS. 12 and 13. The gear housing 9, fixed to the vehicle body, is integrally equipped with a first housing 11 for housing the axially intermediate portion of the rack shaft 7, a second housing 12 for housing the tip half section of the pinion shaft 6, and a third housing 13 for housing the pressing means 10. The pinion shaft 6 has pinion teeth 14 at the portion near the distal end of the outer peripheral surface thereof. The pinion shaft 6 having this configuration is supported by a pair of rolling bearings 15, 16 so as to be only rotatable with respect to this second housing 12 while the tip half section thereof is in a state of being inserted into the inside of the second housing 12.
The rack shaft 7 has rack teeth 17 at a portion near one axial end thereof on the front face. The outer peripheral surface of the rack shaft 7 is a cylindrical face except for the portion in which the rack teeth 17 are formed. In other words, the cross-sectional shape of the outer peripheral surface of the rack shaft 7 is circular at the portions away from the rack teeth 17 in the axial direction, and at the portion in the axial direction in which the rack teeth 17 are formed, the portion corresponding to the rack teeth 17 is linear and the rest is arc-like. The rack shaft 7 having this configuration is supported in the first housing 11 so as to be displaceable in the axial direction via a pair of rack bushings 18, 18 in a state in which the axially intermediate portion is inserted into the inside of the first housing 11 and the rack teeth 17 are engaged with the pinion teeth 14.
The pressing means 10 is housed inside the third housing 13, and has a pressing member 19 and a spring 20. Furthermore, the pressing face of the pressing member 19, that is, the distal end face thereof, is made constant with the portion of the rear face of the rack shaft 7 on the opposite side of the pinion shaft 6 across the rack shaft 7 so that the rack shaft 7 can slide in the axial direction. With this state, the pressing member 19 is elastically pressed against the rear face of the rack shaft 7 using the spring 20. Hence, a preload is given to the engagement portion between the pinion teeth 14 and the rack teeth 17, whereby noise is suppressed from being generated from this engagement portion and the operation feeling of the steering apparatus is improved. The pressing member 19 is wholly made of such a low friction material as described above or has a low friction material layer on the pressing face making slide contact with the rear face of the rack shaft 7.
The front end portion of the intermediate shaft 4 is connected to the base end portion of the pinion shaft 6 via the universal joint 3. The base end portions of the tie rods 8, 8 are connected to the axial end portions of the rack shaft 7 via spherical joints (ball joints) 21, 21. Each of the spherical joints 21, 21 has a spherical body 29 and a cup-shaped joint housing 30 rotatably holding the spherical body 29 inside. Furthermore, the spherical bodies 29 are fixed (integrated or connected and fixed) to the base end portions of the tie rods 8, 8, and the joint housings 30 are securely connected to the end portions of the rack shaft 7 with screws or the like.
According to the steering apparatus configured as described above, when the driver operates the steering wheel 1, the rotation of the steering wheel 1 is transmitted to the pinion shaft 6 via the universal joints 3, 3 and the intermediate shaft 4. As a result, the rack shaft 7 is displaced in the axial direction, whereby the tie rods 8, 8 are pushed and pulled, and the desired steering angle is given to the left and right wheels to be steered.
It has been widely known that producing the rack shaft of the rack-and-pinion steering gear unit to be incorporated into the above-mentioned steering apparatus by bending a metal plate, such as a steel plate, can suppress the production cost of the rack shaft and can reduce the weight thereof according to the descriptions in Patent Documents 1 to 4. FIG. 14 illustrates a method for producing the rack shaft described in Patent Document 3 in the order of the process. First, a long and substantially rectangular (band shaped) blank plate 22 shown in (A) of FIG. 14 is provided by, for example, punching a metal plate having sufficient rigidity, such as a steel plate, by press working. This blank plate 22 is provided, at a portion in the longitudinal direction (in the axial direction of a completed rack shaft 7a and the left-right direction in (A)(a) of FIG. 14), with a narrow portion 23, the dimension of which in the width direction (in the up-down direction in (A)(a) of FIG. 14) is smaller than the dimensions of the other portions.
The blank plate 22 configured as described above is, for example, placed on the upper face of a die and pressed by punching, whereby the widthwise intermediate section thereof is bent and plastically deformed into the shape shown in (B) of FIG. 14 and a first intermediate material 24 is obtained. The cross-sectional shape of a portion (a portion provided with the narrow portion 23) in the longitudinal direction of this first intermediate material 24 is set to a flat U-shape, and the cross-sectional shape of the portion thereof away from the portion provided with the narrow portion 23 in the longitudinal direction is set to a U-shape. Hence, widthwise end edges of the first intermediate material 24 can be made parallel to each other in the longitudinal direction. Next, as shown in (C) of FIG. 14, on the front face (the upper face shown in (C)(a) of FIG. 14) of the portion (the portion whose cross-sectional shape is set to the flat U-shape) in the longitudinal direction of the first intermediate material 24 configured as described above, the rack teeth 17 are formed by press working, whereby a second intermediate material 25 is obtained. Then, the widthwise (circumferential) end edges of the second intermediate material 25 configured as described above are butted to each other (made to contact each other or adjacently opposed to each other) by bending as shown in (D) of FIG. 14, whereby a third intermediate material 26 having a substantially circular pipe shape is obtained. In addition, the widthwise end edges of this third intermediate material 26 are connected to each other by a welded section 27 as shown in (E) of FIG. 14, whereby a fourth intermediate material 28 is obtained. Furthermore, as shown in (F) of FIG. 14, the axially intermediate portion and the end portions of this fourth intermediate material 28 are drawn, and screw holes for screw fixing the male screw portions provided at the base portions of the joint housings 30, 30 (see FIG. 12) of the spherical joints 21, 21 are formed on the inner peripheral surfaces of the axial end portions, whereby a rack shaft 7a having a substantially cylindrical pipe shape is obtained.
With the steering gear unit incorporating the rack shaft 7a configured as described above, the male screw portion provided at the base portion of the joint housing 30 is screwed into the screw hole (the female screw portion) provided on the inner peripheral surface of the end portion of the rack shaft 7a and is further tightened to securely connect the joint housing 30 of the spherical joint 21 to the end portion of the rack shaft 7a. In this case, the force is applied to this screw engagement area in a direction in which the diameter of the end portion of the rack shaft 7a expands. As a result, a tension stress in the circumferential direction is generated at the end portion of the rack shaft 7a. There is a possibility that this tension stress may concentrate at the welded section 27 in which the circumferential end edges of the rack shaft 7a are joined to each other. Hence, in order that the joint state by the welded section 27 can be maintained properly even in this case and that sufficient connection strength can be secured at the connection between the end portion of the rack shaft 7a and the joint housing 30, it is necessary to strictly control the strength of the welded section 27. Consequently, the production cost increases by the amount of the control required.
Furthermore, at the connection between the rack shaft 7a and the spherical joint 21 (screw engagement between the screw hole of the rack shaft 7a and the male screw portion of the spherical joint 21), a large bending load (impact load) is applied in some cases, for example, due to the running of a wheel to be steered over a curbstone. Furthermore, due to this impact load, there is a possibility that the axial end portion of the rack shaft 7a provided with the screw hole may be deformed in the direction that the inner diameter thereof is expanded, stress concentration may occur at the welded section 27 in which the widthwise end edges thereof are securely connected to each other, and the connection strength between the rack shaft 7a and the spherical joint 21 may become lower.